1. The Field of the Invention
The present invention relates to sprayable polymeric compositions and to methods for coating surfaces with such compositions. More particularly, the present invention is directed to solvent free, aliphatic sprayable polyurea-polyurethane coatings which cure rapidly, remain stable in ultraviolet radiation, and are prepared substantially without solvents.
2. Technology Review
Urethane polymers are generally prepared by reacting a polyol or polyol-based compound with an isocyanate, typically in the presence of a catalyst and a curing agent. Urea polymers are generally prepared by reacting a polyamine or polyamine-based compound with an isocyanate. The reaction between an amine and an isocyanate is sufficiently rapid such that a catalyst is not necessary.
Polyurethane coatings generally exhibit good water resistance, solvent resistance, gloss, and adhesion to metal substrates. Because of their useful physical properties, polyurethanes are widely used as coatings. A major problem with conventional polyurethane coatings is yellowing, loss of gloss, or other degradation upon exposure to ultraviolet light. Urethane ultraviolet light instability is often attributed to the use of aromatic isocyanates, such as MDI (methylene bis(4-phenylisocyanate), TDI (toluene 2,4-diisocyanate), or aromatic chain extenders such as DETDA (diethyl toluene diamine). Therefore, aromatic based polyurethane coatings are generally not recommended for exterior use.
It is possible to add pigments or ultraviolet stabilizers or inhibitors to the urethane coating to reduce the degradation effects of ultraviolet light. However, these additives have limited usefulness and tend to dissipate over time. Moreover, they add to the costs and complexity of the ultimate product.
Another possible solution to the problem of ultraviolet degradation of polyurethanes is the use of a protective coating. This approach has been used to protect hot tub covers from ultraviolet light. The base coating provides the desired polyurethane properties, such as waterproofing, and the protective coating shields the base coating from ultraviolet light.
Despite its advantages, this solution to the problem of ultraviolet degradation requires two coatings. The need for a second protective coating increases manufacturing costs from both time and materials. Thus, it is preferable to have a single coating system rather than two coating system.
Others have approached the problem of ultraviolet light instability by replacing the aromatic isocyanates with aliphatic compounds. Unfortunately, aliphatic isocyanates are generally slower reacting than aromatic isocyanates. Only a few aliphatic isocyanates possess a sufficiently high reactivity suitable for sprayable coatings. Moreover, in order to spray these polyurethane coatings, it has been necessary to add solvents to the components in order to have a sprayable viscosity.
A significant problem with solvent-based polymeric coatings is the formation of microscopic pinholes and channels in the coating upon evaporation of the solvent. As a result, the structural properties of the polymeric coating are less than ideal. Moreover, these volatile organic solvents are often flammable and toxic to both humans and the environment. In some cases, the solvents may chemically attack the substrate being coated. This is a common problem with solvent-based polymeric coatings applied to urethane foam substrates.
Sprayable polyurea compositions which are 100% solid (i.e., no solvents used) have been used in the reaction injection molding ("RIM") art. In RIM applications, usually two or more components are mixed and injected under high pressure into a mold where the polyurea composition solidifies. Polyurea compositions are typically more abrasion resistant than polyurethane compositions, presumably because of an additional nitrogen bond to the carbonyl carbon. Polyurea compounds are also thermally stronger and dimensionally more stable than polyurethanes.
However, sprayable polyurea compositions are generally more expensive than comparable polyurethane compositions because suitable polyamines needed to form the polyurea are more expensive than suitable polyols. Thus, due to cost constraints, the use of polyurea compositions has typically been limited to products formed in RIM processes.
From the foregoing, it will be appreciated that what is needed in the art are sprayable polyurea-polyurethane coating compositions which possess some of the physical properties of polyurea compositions, yet may be produced at a cost more comparable to polyurethane compositions. Additionally, it would be a significant advancement in the art to provide sprayable polyurea-polyurethane coating compositions and methods using aliphatic constituents such that the coating remains stable in ultraviolet light.
It would be a further advancement in the art to provide sprayable polyurea-polyurethane coating compositions which are substantially free of volatile solvents thereby resulting in a product which is substantially 100% solid. It would be yet another important advancement in the art to provide sprayable polyurea-polyurethane coating compositions which cure very rapidly, enabling the product to be uniformly sprayed on non-horizontal surfaces without running or streaking.
Such sprayable aliphatic polyurea-polyurethane coating compositions and methods are disclosed and claimed herein.